An EL display is one of light-emitting devices that have attracted most attentions as the next generation flat panel display. In a light-emitting element used for a light-emitting device, a thin film containing an organic compound (hereinafter, referred to as an organic thin film) is formed between two electrodes, and current is applied to the electrodes to generate electroluminescence. Electrons injected from a cathode and holes injected from an anode are recombined in an organic thin film to form a molecular exciton, and when the molecular exciton returns to a ground state, photons are emitted, in other words, light is emitted. At this time, one electron and one hole are recombined to form one molecular exciton. When the molecular exciton is deactivated to the ground state with luminescence, one photon of a wavelength corresponding to energy difference between the excited state and the ground state is emitted (which is called a radiation process).
In a conventional organic EL element, an organic thin film is sandwiched by a pair of an anode and a cathode. Therefore, a ratio of the number of holes and electrons injected from the anode and the cathode (hereinafter, also referred to as carriers in the case where holes and electrons are not distinguished) and the number of emitted photons, in other words, the quantum efficiency is not higher than 1.
As one of methods for solving such a constraint, there is proposed a method for forming an intermediate conductive layer in an element, in addition to the cathode and anode (Reference 1: Japanese Patent Laid-Open No. H11-329748, pp. 6 to 7 and FIG. 3).
According to Reference 1, an organic EL element is proposed, in which an Alq:Li layer 13 including Alq (Al complex of 8-hydroxyquinoline) and Li, and an In—Zn—O (indium.zinc oxide) layer 14 are formed in an element sandwiched by a pair of an anode 11 and a cathode 12, and organic layers 15 and 16 are formed on the opposite sides, namely, the anodes side and the cathode side of the layers (FIG. 14). Note that reference numeral 17 denotes an interlayer insulating film. Here, the layer in which the Alq:Li layer and the In—Th—O layer are stacked is defined as an intermediate conductive layer. In an organic EL element having such a structure, holes and electrons are injected from an anode and a cathode, respectively, as in a general organic EL element. The holes and electrons are transported to the cathode side and the anode side, respectively. The characteristic point here is that carriers are supplied also from the intermediate conductive layer. In other words, electrons are injected into the organic layer 15 from the anode side (Alq:Li layer 13 side) of the intermediate conductive layer, and at the same time, holes are injected into the organic layer 16 from the cathode side (In—Zn—O layer 14 side) of the intermediate conductive layer. In the organic layer 15 on the anode side, holes injected from the anode 11 and electrons injected from the intermediate conductive layers 13 and 14 are recombined to generate a molecular exciton, which leads to luminescence (light-emission). Similarly, in the organic layer 16 on the cathode side, electrons injected from the cathode 12 and holes injected from the intermediate conductive layers 13 and 14 are recombined to generate a molecular exciton, and light-emission is obtained by the radiation process of the molecular exciton.
By the above described mechanism, this organic EL element can provide almost the same effect as the case where general organic EL elements are arranged serially. In other words, although the driving voltage of the organic EL element becomes almost twice higher than a general organic EL element, the number of obtained photons with the same current density becomes twice more than that of the general organic EL element. Therefore, the quantum efficiency almost doubles. It is also possible that the quantum efficiency can be further increased by applying this method. For example, two intermediate conductive layers and three organic thin films are arranged alternately, thereby obtaining almost the same effect as that obtained when three organic EL elements are arranged serially. Therefore, although the driving voltage becomes almost three times higher, the quantum efficiency becomes almost three times higher at the same time. Consequently, a high-luminance organic EL element can be provided.
The requirement for realizing this idea is that the intermediate conductive layer sandwiched by organic layers is transparent. A material that can be used as the intermediate conductive layer is limited so as to fulfill this requirement. Until now, there have been proposed various materials. However, materials that have been used actually are limited to the followings: 1: a stacked layer of a mixed layer containing an electron transporting compound such as Alq or bathocuproin (BCP) and an electron injecting compound such as an alkali metal and a transparent electrode such as ITO (indium tin oxide) or IZO (indium zinc oxide); 2: a stacked layer of the mixed layer containing the electron transporting compound and the electron injecting compound and a metal oxide; 3: a stacked layer of the mixed layer containing the electron transporting compound and the electron injecting compound and an electron-accepting organic compound, and the like.
By using the above described materials, the intermediate conductive layer can keep transparency; however, the material is largely limited and a manufacturing process of an element becomes also complicated. For example, according to the method described in Reference 1, in general, an organic layer is formed by a vacuum vapor deposition method. By this method, a mixed layer of an electron transporting compound and an electron injecting compound can be easily formed on the organic layer. However, a transparent electrode such as ITO or IZO cannot be formed by a vacuum vapor deposition method and formed by a sputtering method. Therefore, such transparent electrodes should be formed after an element is transferred into a chamber for forming a film by a sputtering method from a chamber for an evaporation method, and thus the manufacturing process becomes complicated.
Further, as for the technique related to Reference 1, there are known an organic EL (electroluminescence) element, as shown in References 2 (Japanese Patent Laid-Open No. 2003-45676 p. 1 FIG. 3) and 3 (Japanese Patent Laid-Open No. 2003-272860 p. 1 FIG. 8), in which a plurality of light-emitting units are separated by one layer forming an equipotential surface, and an organic EL element having a structure in which a conductor thin film layer is interposed between two organic EL layers as shown in Reference 4 (Japanese Patent Laid-Open No. 2003-264085 p. 11 FIG. 7).